The present invention relates to an improved fluid loss control additive composition which is particularly useful with drilling fluids which contain one or more mixed metal layered hydroxide compounds. Here after the term "drilling fluid" shall be taken to refer specifically to fluids containing at least one mixed metal layered hydroxide compound. Such drilling fluids may also contain a variety of commonly used chemicals such as, but not limited to, weighting agents, defoamers, shale inhibitors, corrosion inhibitors, and the like.
The use of mixed metal hydroxide materials as gelling agents in drilling fluids is known from the prior art, J. L. Burba et al U.S. Pat. Nos. 4,664,843 and 4,790,954, the teachings of which are expressly incorporated herein.
The Burba et al patents disclose a new and useful material composition for use in drilling fluids. However, when they are used in drilling situations requiring a fluid loss control additive, difficulties may be encountered. For example, the use of certain carboxymethyl cellulosic polymers causes somewhat detrimental effects on the rheology of the drilling fluid, e.g., lower shear viscosity resulting in decreased solids transporting capacity. Commonly available unsubstituted starches which are used regularly in conventional drilling fluids are compatible. They are, however, only partially effective, and ferment readily in aqueous suspensions of bentonite clays containing mixed metal layered hydroxides.